Methods for forming coatings on various kinds of parts have been applied in a wide variety of industrial applications. Coating the parts improves the surface performance of the parts and imparts various functions to the surfaces of the parts. In addition, coating is very important for improvement of appearance of parts and of the products which contain them. Therefore, coating techniques are required to have high reliability. Beside satisfying these requirements, reducing the costs of coating is an important object. These costs should be low enough that they do not account for a significant part of the production costs.
The following are examples of the major resin coating methods used currently:
(1) Electrodeposited plating--In this method, parts are dipped in a liquid in which a charged resin powder is suspended. Voltage from an outside power source is applied to the parts so that the charged powder is attracted thereto. Thus, the parts are coated with the resin powder. Subsequently, the resin powder is heated so that it becomes fused and/or crosslinked, thereby forming a strong, continuous coating on the parts. PA1 (2) Electrostatic coating--In this method parts are subjected to the application of a voltage in a space in which a electrically charged resin powder is dispersed so that the resin powder is attracted to the parts, thereby forming a resin powder coating thereon. Subsequently, the coating is heated so that it becomes fused and/or crosslinked, and a strong and continuous coating is thus formed. PA1 (3) Spray coating--In this method a resin is diluted with a solvent and sprayed on the parts to form a coating. Subsequently, the solvent is vaporized and the coating is fused and/or crosslinked. PA1 (4) Dip painting--In this method parts are dipped in a resin liquid with a low viscosity, or a resin liquid in which the high viscosity is reduced by diluting with a solvent, so that the resin is deposited on the part surfaces. Subsequently, the deposited resin is fused and/or crosslinked so as to form a coating. PA1 (1) Electrodeposition coating: PA1 (2) Electrostatic coating: PA1 (3) Spray coating: PA1 (4) Dip painting--The dip painting method, in which a great number of parts can be plated simultaneously and in a short time, is most efficient and low in cost. It does not suffer from such problems as (a) and (b) of electrodeposition coating, or from such problems as (a) and (b) of spray coating. However, in this method, dripping and sagging of the liquid after the dipping are unavoidable. Or, in other cases, the liquid barely covers the parts or the liquid coverage is extremely thin in some regions. This method is therefore far less reliable as a coating method than the other methods discussed above.
These conventional coating methods have the following problems:
(a) It is necessary to attach the parts to electrodes. PA2 (b) The coating is not formed on the area to which the electrodes are attached. A "touch-up" process is necessary for such area to be coated, in which the part is covered with resin after the coating-formation. In both cases above, a lot of hand labor or the installation of robots which are capable of complex movements is required, which results in high surface treatment cost. PA2 (c) In addition, used liquid from the electrodeposition coating process must usually be treated as industrial waste. PA2 This method involves the same problems as (a) and (b) for electrodeposition coating and further requires the use of large-scale equipment for dust-prevention and prevention of dust explosion, because powder scattering may cause dust explosion. PA2 (a) Handling of the spray-guns requires considerable training and skill. Handling with robots may be possible, but a complex sequence of movements is required, which leads to high costs of coating. PA2 (b) The film thickness tends to vary depending on the operation of the spray gun. PA2 (c) The parts need to be turned over after spraying one side of the work piece so as to spray the other side. PA2 (d) The resin has to be diluted with a large amount of solvent in order to spray. Pollution prevention techniques should be carried out during the evaporation of solvents after spraying.